1. Field of the Invention
This invention relates to improvements in a control system for an internal combustion engine equipped with an exhaust gas purifying catalyst, and more particularly to a technique for judging as to whether a diagnosis for deterioration of the catalyst is to be carried out or not under the action of a control unit.
2. Description of the Prior Art
Recently lowering the emission levels of noxious components (CO, HC, NOx and the like) of exhaust gas discharged from an internal combustion engine has been eagerly required from view points of environmental pollution. In view of this, a catalytic converter including an exhaust gas purifying catalyst is disposed in an exhaust gas passageway to purify exhaust gas by oxidizing or reducing the above noxious components thereby preventing the noxious components from being emitted to the atmosphere. However, such a catalytic converter is subjected to a high temperature under a reaction heat due to a catalytic reaction, and therefore there arise the following cases: Catalyst components sinter. A catalyst carrier overhears and is damaged. Additionally, the catalyst components are poisoned with impurities or the like contained in fuel or lubricating oil. As a result, there is the possibility that an initial exhaust gas purifying efficiency of the catalyst cannot be maintained and drops with time lapse, deteriorating the catalyst components. If the exhaust gas purifying catalyst is used under such a deteriorated condition, a large amount of the noxious components will be discharged to the atmosphere without being converted to harmless components. Accordingly, it has been eagerly required to carry out a diagnosis for the catalyst to know as to whether the catalyst deteriorates or not in order to previously prevent the noxious components from being emitted to the atmosphere.
Here, for the purpose of diagnosing the exhaust gas purifying catalyst, catalyst deterioration diagnosing devices have been proposed and put into practical use for internal combustion engines. Additionally, an air-fuel ratio feedback control system is well know and has been extensively used. The air-fuel ratio feedback control system includes two oxygen sensors which are provided respectively at the upstream and downstream sides of the exhaust gas purifying catalyst, in which a feedback control is made on an air-fuel ratio of air-fuel mixture to be supplied to the engine, in accordance with detected values of these oxygen sensors.
The catalyst deterioration diagnosing device in combination with the air-fuel ratio feedback control system is disclosed, for example, in Japanese Patent Provisional Publication No. 2-136538 and in Japanese Patent Provisional Publication No. 4-17758. The former diagnosing device of the Publication No. 2-136538 is arranged to make a catalyst deterioration diagnosis in accordance with a time duration from a time of re-opening of fuel injection to a time at which the downstream side oxygen sensor outputs a signal representative of a rich air-fuel ratio, after stopping fuel injection. The latter diagnosing device of the Publication No. 4-17758 is arranged as follows: The output levels of the upstream and downstream side oxygen sensors are memorized both at a fuel injection stopping condition and at a high engine load operating condition (or at a rich air-fuel ratio). A catalyst deterioration diagnosis is carried out by comparing these output levels and corresponding oxygen sensor output levels during the air-fuel ratio feedback control.
Now, exhaust gas purifying catalysts are temporarily lowered in capability when it is subjected to a high and lean (air-fuel ratio) atmosphere. In other words, the capability of such exhaust gas purifying catalysts are lowered but can be restored by allowing the catalysts to be subjected to a high and rich (air-fuel ratio) atmosphere.
However, the above-discussed conventional catalyst deterioration diagnosing devices never take account of the temporary capability lowering while a diagnosing condition (in which a catalyst deterioration diagnosis is made) is set in accordance with a response delay of the downstream side oxygen sensor relative to an output (air-fuel ratio) level change of the upstream side catalyst, owing to an oxygen storage effect of the exhaust gas purifying catalyst. Consequently, there is the possibility of making such an erroneous diagnosis that the exhaust gas purifying catalyst is assumed to be completely deteriorated even though the capability of the catalyst temporarily lowers.